Widely used as tubular metal bodies are electro-resistance-welded tubes which are made by forming a metal sheet into a tubular form by roll forming and joining portions thereof butted against each other by high-frequency welding.
Since the weld of the electro-resistance welded tube has a reduced strength by being thermally affected, the weld is susceptible to fatigue fracture due to stress concentration. The use of electro-resistance-welded tubes in pressure piping systems for passing a high-pressure gas has not been approved presently.
Accordingly, tubes extruded with use of a mandrel (hereinafter referred to as “mandrel-extruded tubes”) or tubes extruded through a porthole die (hereinafter referred to as “porthole die-extruded tubes” or “die-extruded tubes”) appear useful for pressure piping systems.
However, mandrel-extruded tubes are likely to have an uneven wall thickness and moreover have the problem that these tubes are not available with a large diameter and/or in a large length. Another problem encountered is that they can not be obtained if having a complex cross sectional shape. These problems may be overcome with porthole die-extruded tubes, whereas these tubes have the following problem. The die-extruded tube is produced by temporarily separating a flow of metal material from a billet into portions at the port part of a porthole die and joining the separated metal material again at the chamber part. The tube comprises a plurality of components as joined to one another with a plurality of joint portions extending over the entire length of the tube. Since the joint portions are inferior to the tube components in mechanical properties such as strength and elongation, the tube is likely to fracture at the joint portion owing to stress concentration when used in pressure piping.
It is thought that the die-extruded tube can be made usable for pressure piping systems when the joint portions are modified in properties. Various heat treatments are known for the billets to be used for extrusion in order to improve the corrosion resistance of the joint portions of porthole die-extruded tubes (see the publication of JP-A No. 1999-172387).
However, methods of improving the mechanical properties of the joint portions still remain to be developed, and die-extruded tubes have not been placed into use for pressure piping systems.
On the other hand, the high-pressure gas to be contained in pressure vessels like those above-mentioned generally has pressure of about 20 to about 35 MPa at present, whereas the pressure will presumably be raised to about 70 MPa in the future.
A liner already known for use in such pressure vessels is produced from a cuplike blank of aluminum by ironing the body of the blank axially thereof by flow forming to make a headplate portion at each of opposite ends of a hollow cylindrical trunk, closing at least one of the head plate portions to give a larger wall thickness to the head plate portion than the trunk and forming a mouthpiece mount bore in a closure portion provided centrally of the head plate portion (see the publication of JP-A No. 1999-104762, claim 1).
However, this pressure vessel liner has the problem of necessitating a cumbersome machining operation and failing to have an increased length and a larger size.
Also known is a liner for pressure vessels which comprises an extruded tubular body of aluminum and head plates welded to respective opposite ends of the body (see the publication of JP-A No. 1999-104762, FIG. 7). Useful as the extruded tubular aluminum body is a mandrel-extruded tube, porthole die-extruded tube, or the like.
The mandrel-extruded tube or die-extruded tube nevertheless has the problem described with reference to the pressure piping, and the die-extruded tube has not been placed into use as the trunk of the liner for pressure vessels.
An object of the present invention, which has been accomplished in view of the foregoing situation, is to provide a tubular metal body which can be of increased length and greater size and which has outstanding pressure resistance, and a method of producing the same.
Another object of the invention is to provide a pressure vessel liner which can be of increased length and greater size and which has outstanding pressure resistance, and a method of producing the same.